This invention relates generally to high power optical transmission gain mediums used in lightwave communication systems and more particularly to high reliability pumping architectures for fiber amplifiers and fiber lasers used in optical signal amplification in lightwave communication systems.
Rare-earth doped fiber amplifiers have come into prominent use in lightwave communication systems, such as for use in optical long haul terrestrial and undersea communications, distributed local area networks and in satellite communication systems, such as communication service between communication satellites in space orbit around the earth. In these systems, a major utility of the rare-earth doped fiber gain medium, such as a fiber laser or fiber amplifier, e.g., erbium-doped fiber amplifiers (EDFAs), is power boosting such as for optical transmitters, receivers or repeaters to periodically boost the transmitted signal to levels sufficient for its reliable transmission along the entire lightwave communication system. In many of these applications, the optical fiber amplifier is physically remote from the point of initial transmission or from the point of final delivery destination so that a highly reliable communication system between the point of communicative origin and final delivery destination is necessary and required due to lack of ability to easily replace or otherwise correct faulty operating components in remote regions of the system, such as, for example, in the case of remotely located optical repeaters or high power optical transmitters in outer space satellites. One such component is the radiation laser sources used to pump the fiber amplifier. Such high power sources have recently become more available in the form of semiconductor pump lasers which are sufficiently reliable to enable the increasing use of rare-earth doped fiber amplifiers.
It is conventional practice to employ 980 nm or 1480 nm semiconductor laser diodes to pump a fiber gain medium to produce amplified optical signals in the range of about 1530 nm to about 1560 nm. Most commonly a multiple QW semiconducter laser diode operating at 980 nm is used because of its low noise figure and good conversion efficiency. The pump laser wavelength of 980 nm falls within the erbium absorption band of the amplifier. In order to enhance the reliability of the semiconductor laser diode pumped fiber gain medium, the outputs of several laser diodes may be optically coupled to a fiber multimode gain medium as illustrated in U.S. Pat. No. 5,263,036, showing a double clad fiber. Thus, if one laser diode malfunctions or becomes inoperative, the remaining laser diodes can continue to function in the pumping task. Also, the use of multiple lasers permits a decrease in the power requirements of the individual laser diode, increasing reliability without compromising the gain of the amplifier as taught in U.S. Pat. No. 5,287,216. The problem arises, however, that when one, and particularly in the case of more, laser diodes fail, an increase in total power required from the remaining laser diodes is inevitable to prevent any compromise of amplifier gain or output power. One manner of solving this problem is to employ a laser array or laser bar so that multiple emitter outputs are provided from a single monolithic laser source. While such an array can provide high pumping power, if one or more laser emitters in the array malfunction or become inoperative, emitter failure affects the quality of the output of the other emitters in the array since the emitters are closely spaced to provide a single full beam output. Such laser arrays can be replaced by broad area emitter laser diodes where several such broad area lasers together can be employed to achieve high power output for the pumping application. However, such broad area lasers have not proved to be reliable over sufficiently long periods of time, particularly since they are subject to filamentation resulting failure, operate at high temperatures and have a tendency to have larger noise figures. Also, broad area lasers need adequate cooling, such as with thermoelectric coolers, adding to the required system components and adding additional concerns for long term system reliability.
The semiconductor laser diode, used as a pumping source for an optical fiber gain medium which is used itself as a pumping source, is considered the least reliable link in an optical fiber pumping system used in a lightwave communication system because these laser devices have a comparatively shorter lifetime. Fiber amplifiers have greater long term reliability compared to semiconductor laser diodes and their lifetime behavior is determinative of the overall lifetime of the lightwave communication system particularly if the system components are in a location not suited for immediate and efficient replacement, such as in the case of outer space.
What is required for enhancing the lifetime capabilities of lightwave communication systems is to increase the long term reliability of the least reliable link, i.e., improve the long term effectiveness of the entire communication system by improving the semiconductor pumping source effectiveness, reliability and longevity in these systems.
Therefore, it is a primary object of this invention to improve the long term reliability of lightwave communication systems.
It is further object of this invention to improve the long term reliability of lightwave communication systems by providing one or more levels of built-in redundancy in the pump sources for the amplifiers employed in lightwave communication systems.
It is still another object of this invention to provide plural levels of redundancy in a high power pump source for solid state optical gain medium, optical fibers being one such medium, to extend the lifetime operation of a lightwave communication while providing longer term maintenance-free operation.
According to this invention, a high power optical amplifier pumping architecture with built-in redundant reliability for a lightwave communication system comprises plural levels of redundancy in the pump source for solid state gain media employed in the system so that the lifetime operation of the entire communication system is significantly increased. This architecture has particular application in communication systems that are placed in remote locations subject to environmental hardships and lack of ease to repair or replace, such as in remote terrestrial and undersea communication locations or outer space satellite locations. The redundant lightwave communication system of this invention comprises a pump source that includes a plurality of semiconductor segmented laser arrays for pumping a single double clad rare-earth doped fiber providing a high power fiber pump source. In turn, a plurality of such high power pump sources are utilized for respectively pumping a plurality of concatenated optical fiber amplifiers wherein the fiber amplifiers are optically coupled as stages to amplify an injection signal provided as input to the first of such fiber amplifier stages.
A first level of redundancy of this invention deals with redundancy of the laser pumping sources for the lightwave communication system. The pumping source comprises a plurality of segmented laser arrays, each array having plural emitters, channels or groups of the laser segments, employed for pumping a single double clad fiber pump source. A sufficient number of segmented laser arrays are provided as primary semiconductor pump sources for a single double clad fiber pump source so that their total operation can be accomplished below the standard rated output levels of the individual arrays, i.e., a plurality of segmented laser arrays for pumping one of a plurality of fiber pump sources are operated below power rated output. Since the segmented laser arrays are operated below their rated power output, an increase in the power output of the remaining segments can be accomplished to meet the demands of the pumping application at its rated power requirements. If the failure is of sufficient magnitude to render ineffective an entire segmented laser array, the remaining segmented laser arrays are operated at their near rated power output to continue the pumping application at its rated power requirements. In other words, if one or more of the segmented laser arrays fails, then the total power output of the remaining segmented laser arrays may be individually increased to compensate for the power loss due to the failed segmented laser array without exceeding the rated power level of any respective segmented laser array. The subsequent achievement of stable operation at full operating power required for the particular pumping application is indicative that the failure is of limited nature and the pumping can be continued at the rated power require for the fiber pump source. Thus, the life expectancy of the operation of the fiber pump source is substantially extended. Moreover, the failure of a single segmented laser array is a precursor of potential system failure and can be employed as an effective warning that immediate system maintenance is eminent.
An additional aspect of this first level of redundancy deals with redundancy in the form of plural power supplies for multiple laser sources in the lightwave communication system. A plurality of laser sources are provided for pumping a fiber pumping source employing a plurality of power supplies. The arrangement may be such that a plurality of laser pump sources can be driven in series by one of the plurality of pumping supplies so that if any one or more power supplies fails, then the remaining power supplies can provide additional electrical pumping power to accommodate the pumping application at its rated power requirements. Moreover, if anyone of the series connected laser pump sources fails, the connected power supply output power signal can be increased to increase the power output of the remaining serially connected laser pump sources. In the extreme, a separate power supply can be provided to separately drive each laser pump source.
A second level of redundancy deals with redundancy of a plurality of fiber laser pump sources for pumping a plurality of serially connected injection signal fiber amplifiers forming the multistage amplifier system. If one of fiber pump sources should fail, increased pumping power is available from the remaining fiber pump sources via their respectively connected amplifier stages.
A third level of redundancy of this invention deals with plural single mode emitters or channels in segmented laser arrays. A reduction in failure rate due to optical degradation of individual laser diode emitters in such arrays is achieved by using multiple narrow stripe emitters in place of single broad area laser diode emitters having substantially no optical overlap with one another. If the individual emitters or channels are sufficiently separated from one another in each segmented laser array, the possibility of lateral dark line defect propagation across the segmented laser array is substantially reduced, and catastrophic failure of adjacent emitters due to lateral propagation of dark line defects or the result of facet damage, in the form of propagating facet blisters into regions of adjacent emitters, of one or more laser emitters in the segmented laser array will not affect the continued operation of other laser emitters in the same array.
Failure rate of adjacent emitters is increased where dark line defects formed in one emitter which fails propagate laterally into one or more adjacent emitters of the array. The propagation of dark line defects is caused by either or the combination of generated light in the form of photons produced in the active regions of adjacent emitters and the pumping current applied to the emitters. If the light created in the respective emitters does not extend or overlap into adjacent emitters, then lateral dark line propagation will not occur. If the current through a failed emitter continues due to an electrical short, dark line defect lateral propagation can continue into adjacent emitters. If the individual emitters on a segmented array are individually addressable, each with appropriate in-line fusing, then an increase in current through the shorted, failed emitter (which is the most common result of emitter failure) will bring about failure of an in-line fuse and terminate further current flow through the failed emitter.
An additional aspect of this second level of redundancy deals with the individual emitters of the segmented laser arrays being operated below their rated power output so that the reliability of the pumping system is enhanced by other adjacent emitters on the same segmented laser array chip taking on the additional power requirements for pumping the fiber pumping source at required operating level when any one or more segmented laser array emitters fail to operate. Thus, an important aspect of this invention is that the array reliability can be evaluated as though the total required power delivery of the plurality of separate and independent segmented laser arrays are individually independent plural single mode laser sources.
The concept of multiple single laser sources for pumping a single amplifier source is known in the art as exemplified in U.S. Pat. Nos. 5,185,758; 5,202,893 and 5,263,036. However, these applications, in principal, involve the use of a plurality of single sources or arrays for pumping an optical amplifier device, such as a fiber amplifier or laser rod, for the purpose of achieving high pumping powers with no treatment provided for long term utility and life expectancy. Also, the concept of multiple laser array sources in the form of segments such as plural laser bar sources coupled into a single lens system is known in the art. Examples of single and multiple semiconductor laser array sources are shown in U.S. Pat. Nos. 4,716,568; 4,803,691; 5,193,098 and 5,384,797, assigned to the assignee herein. However, the consideration and treatment of the operation of these laser bar sources for the purpose of enhancing long term operation while maintaining required power levels of operation has not been considered before in depth relative to applications that are not easily adapted to repair or replacement and considerations of long term usage and lifetime.
It is known in the past that optical isolation can be achieved by segmenting the laser source into many individual separate emitters or channels, albeit single mode or multimode. Past experience has been that, in order to achieve single mode operation of the separate emitters, a center-to-center spacing in the case of nonplanar type active region structures had a spacing anywhere between about 5 xcexcm to 20 xcexcm, and in the case of planar type active regions structures, it was thought that the center-to-center spacing of 10 xcexcm was believed sufficient. However, this has been found out that planar type active regions with such center-to-center spacing may not to be sufficient since multimodes are present because some small fraction of optical overlap is still present, although still proper for some applications, such as in the case of xerographic photoreceptor imaging employing a monolithic array of individually addressable laser emitters. Thus, this concept of separation is good for some applications but is not sufficient for others where strict single mode operation is required with the multiple emitters providing the same wavelength of operation acceptable for the absorption band peak of the active element employed in the fiber pumping source. We have discovered that the channel or emitter separation for planar type active region structures must be greater than a 10 xcexcm center-to-center with its output still capable of being optically coupled to a multimode fiber, such as a double clad fiber; otherwise, there is sufficient optical overlap into adjacent channels that will directly effect the character of their operation. By sufficient optical overlap, we mean that the evanescent field of the propagating radiation in one channel extends into the adjacent channel by even less than 1%, affecting the operation of the entire segment. Therefore, the type of separation desired in a redundant lightwave communication system of this invention is zero optical field overlap and this cannot be accomplished unless the center-to-center separation of the segment channels is at least 13 xcexcm. Thus, the optimal range of center-to-center separation between channels in planar type active region structures is about 13 xcexcm to about 15 xcexcm. Insured zero percent isolation between channels may also be achieved with the provision of grooves between the channels which will also prevent lateral dark line propagation between emitters or channels.
With greater separation among individual channels in a laser segment to achieve substantially zero optical field overlap among the channels, the optics employed as well as the coupling between the segmented laser arrays and the input into the fiber pump sources must provide optimum filing. With comparatively larger separation of array emitters, full brightness from segmented laser arrays must be assured. This is accomplished by beam filling optics comprising, for example, a microlens for each emitter. Moreover, efficient focusing of the resulting bright beams from the microlens array can be accomplished by employing a turning mirror array which provides for common size divergence or beam symmetry in both orthogonal beam dimensions.
In connection with this invention, therefore, a plurality of high power, fiber pump sources are utilized each respectively for pumping one of a plurality of concatenated fiber amplifiers. In order to enhance the power of a transmitted signal, such as in the case of a 1.55 xcexcm optical communication transmitter, a system is comprised of a series of optically coupled single mode fiber amplifiers each pumped by a high power double clad fiber laser pumping source having an output wavelength matching the wavelength absorption band of its respective single fiber amplifier. The plural fiber laser pump sources are each pumped by a plurality of segmented laser arrays with either nonplanar type active region structures such as, buried heterostructures, channeled substrate nonplanar active region waveguide structures, or mesa waveguide structures; or planar type active regions structures, such as a channeled substrate planar active region waveguide structures, ridge waveguide structures or an impurity induced disordered waveguide structures with proper interspatial emitter or channel segmentation, both of which provide for redundancy resulting in optimum reliability and power efficiency. If one of the fiber laser pump sources should fail, the system can still continue to operate with the remaining fiber laser pump sources unaffected by adjacent emitter failure, and together providing the required output to meet the gain requirements of the fiber laser or amplifier.
These and other features of the invention are expressed in further detail in the description and discussion of the redundant pumping scheme and its application to a lightwave communication system as illustrated in the accompanying drawings.